The invention relates to a tack gripper for a tack welding gun, a workpiece which can be tack welded to a component by means of a tack welding gun with a tack gripper and a process for connecting a preferably strip-shaped workpiece to a component.
Welding has evolved in mechanical engineering to a production process with broad applications which is adopted in particular in the case of light construction. Welding also affords an excellent opportunity to reduce the assembly cost of components. In the production of motor vehicles, in particular, welding is the main process for body assembly. Workpieces which serve as fastening and/or holding elements are welded on the body.
It is known to arrange, for example, solid bolts on the body by welding. The welding process as such is substantially automated. The positioning of the workpieces, in particular in the vehicle body region, also takes place substantially automatically. Apparatuses which combine a welding device and a supply device for workpieces are also known.
For the tack welding of workpieces, in particular strip-form workpieces, welding guns are provided with tack grippers. These tack grippers are designed in the manner of tongs. As a result, these tack grippers have a plurality of mechanically moving parts. Welding spatter which impairs the operability of the mechanically moving parts of the tack gripper can be formed during the welding process. In addition to such welding spatter, the mechanically moving parts are contaminated so the mechanically moving parts are subject to increased wear in the course of time. It may also happen that the holding force of the tack gripper no longer suffices as the tong-form tack gripper is inaccessible. It has also been found that contamination on the surfaces of the tack gripper adversely affect the reliable supply of current owing to welding spatter and that the desired resultant weld is not therefore always obtained. As a result, the welds have to be reworked under certain circumstances.
Also, similar results are experienced during the tack welding of workpieces, when tong-like tack grippers and tack welding guns are used.
The purpose of this invention is to provide an improved gripper for holding a workpiece as it is presented to a metal body and welded thereto. The gripper may be used to hold the workpiece while the workpiece is completely welded to the body, or only while the workpiece is temporarily attached by localized spot welds. In this case, if necessary, a more extended weld may be performed later in the manufacturing process.
On the basis of the foregoing, an object of the present invention is to develop a tack gripper which has a smaller number of moving parts. The tack gripper is also to be almost self-cleaning in design. A further object is to provide a process by means of which a strip-form workpiece can be reliably and economically connected to a component. A further object of the invention is to design a workpiece in such a way that it can be gripped reliably and positioned exactly by a tack gripper.
The tack gripper according to the invention for a welding gun is distinguished in that the tack gripper is U-shaped in design with two arms connected via a base, at least one free arm of the tack gripper being resilient. The formerly adopted route is abandoned with this design of the tack gripper according to the invention. In contrast to the known tack gripper, the tack gripper according to the invention has at least one resilient free arm instead of mechanically moving parts. With this design of a tack gripper, a frictional joint is created between the tack gripper and a component which is clamped between the arms. The distance between the arms is dimensioned in such a way that the arms are automatically spread during the gripping of the workpiece.
During the gripping process, the surfaces of the arms rub against the surfaces of the workpiece so the surfaces of the arms, which also act as contact surfaces for the welding current to be introduced are cleaned. The cleaning process is repeated when the tack gripper is removed from the workpiece so two cleaning processes invariably take place during one welding process.
Each arm is preferably detachably connected to the base. Therefore, the arms can easily be exchanged. A further advantage of this design is that the distance between the arms can be increased by spacer members arranged between the base and the arm. The tack gripper can therefore easily be adapted to workpieces of various thicknesses.
According to a further idea, it is proposed that at least one arm be designed with at least one hole in its region adjacent to its free end. If several holes are provided, they are aligned in a line extending transversely to the length of the arm. The design of the tack gripper has the advantage that a workpiece can be provided with catch knobs which engage at least partially in a hole in the arm. This allows, on the one hand, reliable transmission of the welding current and, on the other hand, exact positioning of the workpiece in the tack gripper.
Since the arms are provided with holes, they cannot be clogged by contamination or the like.
At least one arm of the tack gripper has, at its free end, a portion which is bent over itself, the portion being arranged between the arms. If holes are provided in the arms, they are formed in the bent over portion. The bent over portion has the advantage of simplifying introduction of a workpiece into the space between the arms or the portions. The bent over portion also increases the spring action of the arms.
The welding current is transmitted through the arms. The electric resistance of the arms should therefore be as low as possible. Bronze can be considered as a suitable material for forming the arms and/or the base. The spring strength or spring rating of the material of which the arms consist can sometimes be considered as too low to produce an adequate holding force. It is therefore proposed that at least one arm be spreadable against an additional spring force. At least one spring element arranged on an arm is provided for this purpose. This spring element can be a spring subjected to bending stress, the spring element being fixed at one end in a region of the arm in which the arm at least partially overlaps the base. A spring element which is a leaf spring is preferred. The leaf spring can also consist of spring steel. To increase the spring force exerted on the respective arm by the spring element it is proposed that the spring element be designed in such a way that it is curved in an arc toward the arm at least at its free end.
The welding current is preferably introduced into the workpiece via the base and the arms. For this purpose, the base is formed by a cuboid carrier having a connection to the welding head of the tack welding gun on one of its end faces. The arms and optionally the spring element or the spring elements are preferably screwed to the cuboid carrier.
In order to center the arms on the carrier, each arm has a groove in which a respective base formed on the carrier engages. The centering of the arms is particularly important if each arm has, in a region adjacent to its free end, at least one hole which cooperates with catch knobs formed on the workpiece.
To minimize the production costs of the tack gripper according to the invention, it is proposed that the arms of the tack gripper be identical in design. The storage of spare arms is therefore also minimized. The leaf springs can be standard springs.
The thermal stress on the tack gripper during the welding process can be considerable. If the tack gripper is used in a handling device, the time interval between two welding processes is sometimes too small to allow adequate cooling of the tack gripper. It is therefore proposed that the tack gripper be designed so as to allow the fastest possible discharge of heat and therefore cooling of the tack gripper. It is proposed for this purpose that the base and/or the arms be provided with at least one cooling fin. If the convective cooling of the tack gripper is not adequate or not fast enough, the base can be designed with at least one cooling duct which is connected to a coolant supply system, instead of cooling fins or in addition to them. The base can have several cooling ducts for this purpose.
If a coolant which is conveyed through the cooling ducts is used, an embodiment in which the coolant is circulated is preferred. This prevents losses of coolant. In order to produce a circuit, it is therefore proposed that the duct or ducts formed in the base be connected to the coolant supply system via a feed conduit and a discharge conduit. The coolants can be protective gas, water and/or air. It is particularly appropriate to use protective gas as coolant as it is required during the cooling process anyway.
The tack gripper according to the invention for a tack welding gun is preferably used for workpieces comprising at least a pair of gripping surfaces grippable by a tack gripper, at least one of the gripping surfaces being designed with at least one catch knob engaging at least partially in a hole in an arm. This results in reliable holding of the workpiece and very good electrical contact between the workpiece and the tack gripper. The catch knobs are preferably formed by embossment. The height of the catch knobs is preferably between 0.3 and 0.5 mm. The height is adequate to ensure a secure hold in the holes. However, the knobs are not so markedly embossed that they are still visible after painting of the workpiece.
According to a further idea of the invention, a process for connecting a strip-form workpiece, preferably a workpiece comprising catch knobs on the gripping surfaces, is proposed with a component in which a tack gripper of a tack welding gun grips and frictionally holds a portion of metallic band. A separating device severs a strip of predetermined length from the band. The strip forms a workpiece. The workpiece held by the tack gripper is initially positioned on the component and then welded thereto. On termination of the welding process, the tack gripper is removed from the workpiece. The tack gripper can grip another portion in a metallic band. The procedure is repeated. The process is automated in this way. Moreover, the tack gripper formed on the tack welding gun avoids the need for further feed arrangements.
According to a further aspect of the invention, the gripper can be a tack gripper and the welding gun can be a tack welding gun.
Further advantages and features will be described with reference to an embodiment of a tack gripper and a workpiece.